Cement accelerating admixture

ABSTRACT

An accelerating admixture for use with sprayable cementitious compositions such as shotcrete comprises a mixture of (a) amorphous aluminium hydroxide and/or aluminium hydroxysulphate, and (b) at least one amine, preferably an alkanolamine. In a preferred embodiment, the admixture also comprises an aqueous dispersion of at least one polymer which comprises pendant carboxylic moieties.

This invention relates to cementitious spraying compositions and toadmixtures for use therein.

Cementitious compositions for application to a substrate by spraying,for example, "shotcrete", must be capable of being sprayed in the firstplace (that is, they must retain sufficient fluidity), and on contactingthe surface, they must set quickly (generally within a few minutes) togive a stiff layer to which further layers may quickly be applied. Theart-recognised way of achieving this has been the addition at the pointof spraying (for example, by injecting into a spray nozzle) of anaccelerator in the form of an aqueous solution. The accelerators usedare generally strong alkalis such as sodium aluminate, sodium orpotassium hydroxide or carbonate and triethanolamine (see, for example,"Concrete Admixtures Handbook", ed. Ramachandran (Noyes, 1984)).

The nature of such substances makes for unpleasant working conditions onspraying sites, and there has long been a need for an effective sprayingaccelerator which has less of the caustic character of the knownaccelerators. In a recent development, accelerators comprising amorphousaluminium hydroxide have been proposed. These have the advantage of lowalkalinity and high performance. Typical examples of accelerators havebeen blends of amorphous aluminium hydroxide with gypsum or variouswater-soluble salts.

Accelerators applied at the nozzle are in liquid form. This means that,in the case of aluminium hydroxide, the solid accelerator must bedispersed in water. This can be done on site, but it is much moreconvenient if such an accelerator can be supplied in liquid form. Thishas hitherto not been possible with aluminium hydroxide, dispersions ofwhich are not storage-stable for more than a few days.

It has now been found that a particular combination of aluminiumhydroxide with another material not only has excellent storage stabilitybut also is particularly effective as an accelerator for sprayingcementitious compositions. There is therefore provided, according to thepresent invention, an accelerating admixture for use in the spraying ofcementitious compositions, comprising (a) amorphous aluminium hydroxideand/or aluminium hydroxysulphate, and (b) at least one amine.

There is additionally provided a method of preparing a storage-stable,amorphous aluminium hydroxide and/or aluminium hydroxysulphate-basedaccelerating admixture by the dispersion of amorphous aluminiumhydroxide and/or aluminium hydroxysulphate in water, there beingadditionally added at least one amine.

In the particular circumstances of this invention, aluminiumhydroxysulphate is often a better accelerator than aluminium hydroxideand is therefore preferred. It is possible to use a mixture of aluminiumhydroxysulphate with aluminium hydroxide and/or aluminium sulphate, butit is preferable to use the hydroxysulphate alone. A typicalcommercially-available aluminium hydroxysulphate is "Gecedral" (trademark) L (ex Guilini Chemie GmbH, Ludwigshafen/Rhein, Germany).

The amines for use in this invention should be liquid or soluble inwater. A highly preferred class of amines is that of the alkanolamines.The preferred alkanolamines are dialkanolamines and trialkanolamines,more preferably diethanolamine and triethanolamine. Most preferred is amixture of diethanolamine and triethanolamine.

In an admixture according to the invention, the aluminium hydroxideand/or hydroxysulphate is combined with the alkanolamine in the weightratio of from 1:10-1000:1, preferably from 1:0-100:1, more preferablyfrom 1:1-0:1. When the preferred mixture of diethanolamine andtriethanolamine is used, the ratio of diethanolamine to triethanolamineis from 1:1 to 0:1.

The constituents of the admixture may be added to a spray nozzleseparately or preferably as a pre-mixed blend. The invention thereforealso provides an accelerating admixture which is a mixture of theaforementioned constituents. In either case the weight ratio of theconstituent on cement is from 0.5-10% for the aluminiumhydroxide/hydroxysulphate and from 0.001 to 5%, more preferably from0.001-2% or 0.01-5% for the amine.

In an especially preferred embodiment of the invention, the acceleratingadmixture of the invention may be combined with certain other materialsto provide particularly outstanding results. The invention thereforealso provides a spraying admixture which comprises a combination of anaccelerating admixture as hereinabove defined and an aqueous dispersionof at least one polymer which comprises pendant carboxylic moieties.

The polymers useful in this aspect of the invention are any polymerswhich bear suitable pendant carboxylic moieties. The carboxylic moietymay be a carboxyl group, and any polymer having such groups andcomplying with the other parameters hereinunder described will work inthis invention. Examples include poly(acrylic) and poly(methacrylic)acids and mixtures thereof and copolymers of (meth)acrylic acid withother comonomers. However, preferred polymers are of the "comb" type,that is, polymers comprising a polymer backbone (whose chemical natureis irrelevant to the working of the invention) with carboxylgroup-containing moieties pendant therefrom. The moieties may be up to100 units long and may comprise monomer residues of, for example(meth)acrylic acid, maleic acid or fumaric acid. It is preferred thatthe moieties be completely made up of (meth)acrylic acid residues.

A preferred backbone is a random addition polymer, one of the monomersof which comprised, prior to polymerization, more than one, andpreferably two, polymerizable double bonds. A preferred monomer withmore than one double bond is butadiene and a preferred copolymer is astyrene-butadiene copolymer. To such a copolymer (which may have pendantand/or in-chain polymerizable double bonds) carboxyl group-containingmonomers such as acrylic acid, methacrylic acid, maleic acid or fumaricacid may be graft copolymerized. Examples of preferred copolymers arestyrene-butadiene copolymerized with acrylic acid andstyrene/butadiene/maleic (or fumaric) acid terpolymer.

The polymers of this invention have molecular weights (weight average)in excess of 20,000 and are solids at room temperature. They also have alimited solubility in water, being at best dispersible (never completelysoluble), but they are highly soluble in alkaline media (such as a fluidcementitious mix). In the case of poly(meth)acrylic acid and polymerswhich are inherently soluble in water, it is possible to prepare suchpolymers in such a form that the necessary solubility is achieved, forexample, particulate form. They are added to the cementitious mix as anaqueous dispersion of less than 5% solids by weight.

Typical examples of suitable commercial materials are those availableunder the trade mark "Synthomer" (ex Synthomer Chemie GmbH,Frankfurt/Main, Germany). An especially suitable example is "Synthomer"9523 (a methacrylic acid-styrene-butadiene copolymer). Another suitablecommercial product is "Polyacryl" AD (ex Polygon Chemie AG, Olten,Switzerland)

The aqueous dispersion is added to the cementitious composition at thenozzle, either separately from or in a mixture with the acceleratiingadmixture hereinabove described. The proportion of polymer present isfrom 0.01 to 3% by weight on cement.

When the spraying admixture comprises an aqueous polymer dispersion ashereinabove described, the admixture must be acid, preferably with a pHof 6 maximum. This may be achieved by the addition of an acid, which maybe chosen from any suitable strong organic or inorganic acid, which isadded in sufficient quantity to achieve the desired pH. In addition, theacid must not react adversely with any of the components of thecementitious composition or the admixture to an unacceptable degree.Reactivity will vary, depending on the individual acid and thecomponents present, but the selection of a suitable acid is not aproblem for the skilled person. It is preferred to use a strong mineralacid, and nitric acid has been found to work particularly well in mostcircumstances.

It has been found that the use of an aqueous polymer dispersion ashereinabove defined in conjunction with an accelerating admixture ashereinabove described enhances the performance of a sprayed cementitiouscomposition considerably. In particular, it appears to enhance adhesionof the sprayed composition to the substrate in the important few minutesafter spraying when the hardening of the composition is not sufficientlyadvanced to ensure good adhesion.

A preferred spraying admixture has the following composition (drycontents, by weight, the remainder of the formulation to 100% beingwater):

    ______________________________________                                        aluminium hydroxide/hydroxysulphate                                                                   20%-60%                                                 alkanolamine 0.01%-25%                                                        polymer dispersion 0%-20%                                                     acid 1%-20%                                                                 ______________________________________                                    

A particularly preferred composition has the composition (dry contents,by weight, the remainder of the formulation to 100% being water)

    ______________________________________                                        aluminium hydroxysulphate                                                                            40%-60%                                                  triethanolamine 0.01%-20%                                                     diethanolamine 0%-5%                                                          acrylic copolymer dispersion 10%-20%                                          nitric acid 5%-20%                                                          ______________________________________                                    

The compositions and processes of this invention are useful in thespraying of cementitious composition by both the "wet" and "dry"methods, but they work better in the "wet" method where water is addedat the mixing stage prior to pumping. The invention therefore provides asprayable cementitious composition which comprises an admixture ashereinabove defined.

The sprayable compositions according to this invention may also compriseany of the wide range of other admixtures known to be useful insprayable cementitious compositions, used in art-recognised quantities.One particularly useful category of admixture is that of theplasticisers and superplasticisers; these can assist in the achievementof a desirable water/cement (W/C) ratio. A further class of usefuladmixtures are set retarders. Any of the known types may be used, aparticularly useful class of materials being phosphonic acid-basedmaterials, in particular those described in U.S. Pat. No. 4,964,917

A particular feature of the invention is the excellent storage stabilityof the admixtures according to the invention. If aluminium hydroxideand/or aluminium hydroxysulphate is dispersed in water, the viscosityrises rapidly to a value which renders the mixture unusable very quickly(typically LVT Brookfield viscosity 100,000+ mPas (Spindle No.4 at 1.5rpm) a few days after mixing and completely solid after one week). Incontrast, an admixture according to the-present invention stored at 20°C. remains usable for 6 months. Thus, it is possible to prepareadmixtures well in advance of use and have them immediately ready, thusreducing the need to mix on site.

Sprayable cementitious compositions which utilize this invention formthick layers which "set up" quickly and can be resprayed without anyproblems. Rebound characteristics are particularly good, as are bothstrength development and long term strength. The invention thereforeprovides the use of a mixture of aluminium hydroxide and/orhydroxysulphate and at least one amine as an accelerating admixture forcementitious compositions which are to be sprayed. The invention furtherprovides a method for the spraying of a cementitious composition on to asubstrate from a spray nozzle, wherein there is added to the compositionat the nozzle an accelerating admixture as hereinabove defined.

The invention is further described with reference to the followingexamples in which all parts are expressed by weight.

(a) Preparation of spraying admixture

The following materials are blended under low shear to form ahomogeneous mixture:

    ______________________________________                                        water                   55 parts                                                aluminium hydroxysulphate.sup.1 17.9                                          triethanolamine  7.7                                                          diethanolamine  1.55                                                          65% nitric acid  7.7                                                          aqueous polymer dispersion.sup.2  6.15                                      ______________________________________                                         .sup.1 "Gecedral" L ex Guilini Chemie                                         .sup.2 "Synthomer" 9523 ex Trub Chemie                                   

(b) Testing of spraying admixture

(i) A spraying concrete mixture is prepared with a W/C ratio of about0.45. The mixture is sprayed on to a substrate through a nozzle with adosing at the nozzle with the spraying admixture of 4.8% by weight oncement. It is found that the sprayed concrete goes matt immediately(signifying immediate setting) and that a layer of thickness in excessof 25 cm. can quickly be built up with very little rebound. A measurablestrength develops after only 6 minutes, and after 16 hours the strength(measured at a number of different points) is between 7 and 18.8 N/mm².

(ii) The procedure of (i) above is repeated with a dosing of 3.8%. Inthis case, the thickness readily achievable is about 20 cm. and earlystrength development becomes measurable after about 20 min. After 16hours, strength is measured and found to be between 14.7 and 19.6 N/mm².

I claim:
 1. An accelerating admixture for use in the spraying ofcementitious compositions, comprising an additive selected from thegroup consisting of (a) amorphous aluminum hydroxide, aluminumhydroxysulphate, or mixtures thereof and (b) at least one amine.
 2. Anaccelerating admixture according to claim 1, wherein the constituents(a) and (b) are components of a single mixture.
 3. An acceleratingadmixture according to claim 1, wherein aluminum hydroxysulphate is usedand aluminum hydroxide is excluded.
 4. An accelerating admixtureaccording to claim 2, wherein aluminum hydroxysulfate is used andaluminum hydroxide is excluded.
 5. An accelerating admixture accordingto claim 1, wherein the amine is an alkanolamine, said amorphousaluminum hydroxide, aluminum hydroxysulphate, or mixtures thereof arecombined with the alkanolamine in the weight ratio of 1:10 to 1000:1. 6.An accelerating admixture according to claim 2, wherein the amine is analkanolamine, said amorphous aluminum hydroxide, aluminumhydroxysulphate, or mixtures thereof are combined with the alkanolaminein the weight ratio of 1:10 to 1000:1.
 7. An accelerating admixtureaccording to claim 3, wherein the amine is an alkanolamine, saidamorphous aluminum hydroxide, aluminum hydroxysulphate, or mixturesthereof are combined with the alkanolamine in the weight ratio of 1:10to 1000:1.
 8. An accelerating admixture according to claim 4, whereinthe amine is an alkanolamine, said amorphous aluminum hydroxide,aluminum hydroxysulphate, or mixtures thereof are combined with thealkanolamine in the weight ratio of 1:10 to 1000:1.
 9. An acceleratingadmixture according to claim 5 wherein the alkanolamine is selected fromthe group consisting of dialkanolamine and trialkanolamine.
 10. Anaccelerating admixture according to claim 5, wherein the alkanolamine isselected from the group consisting of diethanolamine, triethanolamine,or mixtures thereof.
 11. An accelerating admixture according to claim 1,wherein the admixture additionally comprises an aqueous dispersion of atleast one polymer containing pendant carboxylic moieties.
 12. Anaccelerating admixture according to claim 2, wherein the admixtureadditionally comprises an aqueous dispersion of at least one polymercontaining pendant carboxylic moieties.
 13. An accelerating admixtureaccording to claim 3, wherein the admixture additionally comprises anaqueous dispersion of at least one polymer containing pendant carboxylicmoieties.
 14. An accelerating admixture according to claim 4, whereinthe admixture additionally comprises an aqueous dispersion of at leastone polymer containing pendant carboxylic moieties.
 15. An acceleratingadmixture according to any one of claims 11-14, wherein the polymercontaining pendant carboxylic moieties is a comb-polymer.
 16. Asprayable cement composition which comprises the admixture according toany one of claims 1-14, wherein the amorphous aluminum hydroxide,aluminum hydroxysulfate, or mixtures thereof component is present in theamounts of 0.5 to 10% and the amine component is present in the amountsof 0.001 to 5%.
 17. A sprayable cement composition which comprises theadmixture according to any one of claims 1-14, wherein the amorphousaluminum hydroxide, aluminum hydroxysulfate, or mixtures thereofcomponent is present in the amounts of 0.5 to 10% and the aminecomponent is present in the amounts of 0.001 to 5%, said admixturefurther comprises an aqueous dispersion of a comb-polymer.
 18. Aspraying admixture in dry contents by weight, water to 100%, comprising:

    ______________________________________                                        aluminium hydroxide/hydroxysulphate                                                                   20%-60%                                                 alkanolamine 0.01%-25%                                                        polymer dispersion 0%-10%                                                     acid 1%-20%.                                                                ______________________________________                                    